This invention relates generally to intersubband (ISB) superlattice (SL) light emitters and, more particularly, to quantum cascade (QC) SL lasers that have essentially flat minibands and spatially symmetric wavefunctions.
In an ISB SL laser the optical transition takes place between an upper miniband (i.e., energy states near or at the bottom of that band) and a lower miniband (i.e., energy states near or at the top of that band). In order for these lasers to properly function a flatband condition of the upper and lower minibands must exist; i.e., two conditions should be met: (1) macroscopic alignment of each radiative transition (RT) region with adjacent injection/relaxation (I/R) regions, and, more importantly, (2) microscopic alignment of the upper laser energy levels within each RT (and similar alignment of the lower laser levels). However, in the presence of an applied field (e.g., the external bias applied transverse to the layers to induce lasing) the quantum states, from quantum well (QW) layer to QW layer, shift to higher and higher energies in the direction of the field if an SL of essentially identical QW regions is used. This problem is addressed in U.S. Pat. No. 6,055,254 granted to F. Capasso et al. on Apr. 25, 2000 (hereinafter the Capasso 55 patent), which is assigned to the assignee hereof and which is incorporated herein by reference. The Capasso 55 patent describes an ISB SL laser in which the internal electronic potential is pre-biased by varying the SL period so as to achieve an essentially flat profile, on average, of the upper and lower minibands despite the presence of an applied electric field in the SLs. FIG. 2 of Capasso 55 (nearly identical to FIG. 2 herein) illustrates the essentially flatband condition in the presence of an applied field; i.e., energy states (as represented by their wavefunctions) near or at the bottom of miniband 2 in each RT region (e.g., RT 14.5) are fully spread across the RT region, as are the energy states at or near the top of miniband 1. In each of the upper minibands 2 the wavefunction is essentially spatially symmetric with respect to a vertical plane through the midpoint (hereinafter the midplane) of the unipolar radiative transition (RT) region. In contrast, in each of the lower minibands 1 the wavefunction is significantly spatially asymmetric (illustratively having larger magnitude lobes to the right side of the midplane). The asymmetry increases as the width of the miniband decreases. One effect of this asymmetry is a lower optical dipole matrix element and hence a less efficient emitter.
In principle, the Capasso 55 design does permit the two wavefunctions at the edges of the minigap (i.e., the two energy levels or states involved in the optical transition) to be symmetric, but only in relatively narrow ranges of wavelength and electric field. However, making the wavefunctions of those two states symmetric leaves no additional degrees of freedom needed to optimize and/or control many other design issues. For example, the following parameters would be fixed: the shape of the wavefunctions and the energy position of the rest of the states in the two minibands, which may affect, among other things, injection and extraction of electrons from the minibands or optical absorption within a miniband.
Thus, a need remains in the art of ISB SL emitters for a design that provides not only essentially flat minibands but also provides essentially spatially symmetric wavefunctions (for at least the two wavefunctions involved in the optical transition) with independent control of the applied electric field, the desired wavelength of operation, the shape of the wavefunctions, and the energy position of the rest of the states in the minibands. By independent control we mean the ability to choose the value of one parameter within a set of parameters independent of the values of other parameters in the same set. Sometimes we refer to such a choice herein as being arbitrary.
In some applications, there is also a need to achieve these features in an ISB SL emitter in which essentially all of the wavefunctions in the upper and lower minibands are essentially spatially symmetric.
In accordance with one aspect of our invention, the RT regions of an ISB light emitter comprise pre-biased SLs and a multiplicity of split quantum wells (SPQWs). By a SPQW we mean a quantum well that is divided into a multiplicity of sub-wells by at least one barrier layer sufficiently thin that the upper and lower energy states are separated beyond their natural broadening and contribute to different minibands in each RT region. In contrast, adjacent SPQWs are coupled to one another by other barrier layers having thicknesses such that minibands are created across each RT region.
In one embodiment, our invention includes an I/R region between adjacent RT regions, and in another embodiment the I/R regions are omitted (i.e., an injectorless ISB emitter).
Our invention allows at least the two wavefunctions involved in the optical transition (and in one embodiment, essentially all of the wavefunctions in the minibands) to be spatially symmetric and provides an additional degree of freedom that is important for optimizing the energy position of the minibands and hence the injection and extraction of electrons. Combining the freedom to position the energy of the minibands with the ability to arbitrarily choose the electric field enables the injectorless ISB emitter design.